1. Field of the Invention
The present invention relates to a method of making a distribution of energy of a laser beam uniform in a particular region, and to a laser irradiation apparatus (including a laser device and an optical system for guiding a laser beam output from the laser device to a target) for annealing semiconductor film by using a laser beam (hereinafter referred to as laser annealing). And also the present invention relates to a method of manufacturing a semiconductor device manufactured by a method including a process of the laser annealing. In this specification, xe2x80x9csemiconductor devicexe2x80x9d denotes the category of any device capable of functioning by utilizing a semiconductor characteristic, covering electro-optic devices, such as liquid crystal display device and electroluminescent (EL) display devices, and electronic devices including such a kind of electro-optic device as a component.
2. Description of the Related Art
In recent years, studies have been widely made of techniques for performing laser annealing on an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystalline property but not single crystalline, i.e., a polycrystalline or microcrystalline semiconductor film), i.e., a non-single crystalline semiconductor film, formed on an insulating substrate such as a class substrate to crystallize the film or improve the crystallographic characteristics of the film. As the above-described kind of semiconductor film, silicon film is ordinarily used.
Glass substrates are low-priced and having high workability in contrast with quartz substrates conventionally used widely. Because of these characteristics, glass is frequently used as the material of a large-area substrate. This is the reason for making the above-mentioned studies. Laser is favorably used for crystallization because the melting point of a glass substrate is low. Laser enables supply of a large amount of energy only to a non-single crystalline film over a substrate without a considerable increase in the temperature of the substrate.
Conventionally, for crystallization of an amorphous semiconductor film by heat, heating at a temperature of 600xc2x0 C. or higher for ten hours or longer and, preferably, twenty hours or longer is required. An example of a substrate capable of enduring under this crystallization condition is a quartz substrate. A quartz substrate, however, is high-priced and not sufficiently workable. In particular, it is extremely difficult to work quartz into a large-area substrate. Increasing the area of a substrate is an essential factor in increasing the efficiency with which a semiconductor device using the substrate. In recent years, schemes to increase the substrate area for the purpose of improving the production efficiency have been markedly advanced. A substrate size of 600xc3x97720 mm is now becoming a standard with respect to factory lines newly constructed.
It is difficult to work quartz into such a large-area substrate as long as a presently available technique is used. A large-area quartz substrate, if any, must be high-priced and is not industrially usable. On the other hand, glass is an example of a material from which a large-area substrate can be easily made. As a glass substrate, a glass called Corning 7059 may be mentioned. Corning 7059 is markedly low-priced and sufficiently workable and can be easily formed into a large-area substrate. Corning 7059, however, has a strain point of 593xc2x0 C. and cannot be heated at a temperature of 600xc2x0 C. or higher without a problem.
A Corning 1737 substrate having a comparatively high strain point, 667xc2x0 C. is known as one of the existing glass substrates. The result of an experiment made by forming an amorphous semiconductor film on Corning 1737 and maintaining the amorphous semiconductor film at a temperature of 600xc2x0 C. for 20 hours was that there was no such deformation of the substrate as to influence the fabrication process, and the amorphous semiconductor film was crystallized. However, the heating time 20 hours is excessively long if considered as a heating time in a practical production process, and it is desirable to reduce the heating temperature below 600xc2x0 C. from the viewpoint of production cost.
To solve this problem, a new crystallization method has been devised, details of which are as described in Japanese Patent Application Laid-open Hei No. 7-183540. This method will be described briefly below. First, a small amount of an element, e.g., nickel, palladium, or lead is added to an amorphous semiconductor film. For this addition, a plasma processing or deposition method, an ion implantation method, a sputtering method, a solution application method, or the like may be used. After the addition, the amorphous semiconductor film is placed, for example, in a nitrogen atmosphere at 550xc2x0 C. for 4 hours to obtain a polycrystalline semiconductor film having good characteristics. The heating temperature and heating time and other hearing conditions most suitable for crystallization depend on the amount of the added element and the state of the amorphous semiconductor film.
An example of crystallization of an amorphous semiconductor film by heating has been described. On the other hand, crystallization of a semiconductor film by laser annealing can be performed even on a plastic substrate or the like as well as on a glass substrate having a low strain point because laser annealing enables supply of a large amount of energy only to the semiconductor film without a considerable increase in a substrate temperature.
Examples of a laser used for laser annealing are an excimer laser and an Ar laser. As a laser annealing method having the advantage of improving the productivity and mass-producibility, a method is favorably used in which a high-power laser beam obtained by pulse oscillation is processed by an optical system so as to form a spot having the shape of a several centimeters square, or a stripe having, for example, a length of 10 cm or longer along an irradiation plane, and the laser irradiation position is moved relative to the irradiation plane in a scanning manner to perform laser annealing. In particular, a use of a laser beam forming a linear on the irradiation plane (hereinafter referred to as xe2x80x9clinear beamxe2x80x9d) is effective in improving the productivity in contrast with a use of a spot laser beam, because scanning with the linear beam only along the direction perpendicular to the lengthwise direction of the stripe formed by the linear beam may suffice for irradiation of the entire target surface while scanning with the spot laser beam must be performed along each of two directions perpendicular to each other. Scanning along the direction perpendicular to the lengthwise direction of the stripe formed by the linear beam has maximum scanning efficiency. Because of this advantage in terms of productivity, a use of a linear beam obtained by processing high-power laser light with a suitable optical system is now becoming mainstream in laser annealing.
FIG. 2 shows an example of an optical system for processing a laser beam so that the beam forms a stripe on a surface to be irradiated. The optical system also has the function of making the distribution of laser beam energy along the irradiation plane uniform as well as processing the laser beam in the form of stripe. In general, an optical system for making the distribution of beam energy uniform is called a beam homogenizer.
The optical system will be described first with reference to the side view in FIG. 2. A laser beam emitted from a laser oscillator 201 is divided by a cylindrical lens array 202 in a direction perpendicular to the direction in which the laser beam travels. This direction perpendicular to the laser beam traveling direction will be referred to as xe2x80x9cshort-dimension directionxe2x80x9d in this specification. In the example shown in FIG. 2, the laser beam is divided into four. The divided laser beams are converged by a cylindrical lens 204 so as be temporarily combined into one. The beams are thereafter reflected by a mirror 206 and then combined into one on an irradiation plane 208 by a doublet cylindrical lens 207. The doublet cylindrical lens is a lens formed by two cylindrical lenses. Thus, the distribution of energy of the linear beam in the short-dimension direction is made uniform and the dimension of the beam in the short-dimension direction is determined.
The optical system will next be described with reference to the top view in FIG. 2. The laser beam emitted from the laser oscillator 201 is divided by a cylindrical lens array 203 in a direction perpendicular to the direction in which the laser beam travels and also perpendicular to the short-dimension direction. This dividing direction will be referred to as xe2x80x9clong-dimension directionxe2x80x9d in this specification. In the example shown in FIG. 2, the laser beam is divided into seven. The divided laser beams are combined into one on the irradiation plane 208 by a cylindrical lens 205. Thus, the distribution of energy of the linear beam in the long-dimension direction is made uniform and the dimension of the beam in the long-dimension direction is determined.
Each of the above-described lenses is made of synthetic quartz suitable for use with an excimer laser, and an antireflection coating is formed on the surface of each lens to improve transmission of excimer laser light. As a result, the transmittance of each lens with respect to excimer laser light is 99% or higher.
The surface of an amorphous semiconductor film is irradiated with the linear beam processed by the above-described optical system while the beam is gradually shifted in the short-dimension direction so that the irradiated areas overlap, thus performing laser annealing on the entire surface of the amorphous semiconductor film. The amorphous semiconductor film is thereby crystallized or the crystallographic characteristics of the semiconductor film are improved.
The crystalline semiconductor film obtained by the above-described laser annealing is formed of a multiplicity of crystal grains and is therefore called a polycrystalline semiconductor film. Polycrystalline semiconductor films have a markedly high mobility in comparison with amorphous semiconductor films. Therefore, a use of a polycrystalline semiconductor film enables fabrication of a monolithic liquid-crystal electro-optic device (a semiconductor device having thin-film transistors (TFTs) made on one substrate for drive pixel-forming elements and drive circuits), which cannot be realized by using a semiconductor device made by using a conventional amorphous semiconductor film. Thus, polycrystalline semiconductor films have highly advantageous characteristics in comparison with amorphous semiconductor films.
A method of crystallizing an amorphous semiconductor film by performing heating and then performing laser annealing may also be used as well as the above-described method. In some cases, this method is more effective in improving the characteristics of the semiconductor film than that using one of heating and laser annealing for crystallization. To obtain improved characteristics, it is necessary to optimize heating conditions and laser annealing conditions. If a thin-film transistor (TFT) is fabricated by, for example, a well-known method and by using a polycrystalline semiconductor film obtained by the above-described method, the TFT can have remarkably improved electrical characteristics.
Laser annealing is now becoming indispensable for producing a semiconductor film having improved electrical characteristics at a reduced cost. However, the performance of available laser oscillators is not high enough to mass-produce the desired film and there are problems of mass production to be solved, including a problem relating to maintenance of apparatuses used to perform laser annealing. To perform laser annealing of a semiconductor film, at least a laser oscillator, an optical system for making the distribution of energy of a laser beam uniform and for processing the laser beam as desired, and a robot for transporting the semiconductor film are required.
Excimer lasers are often used as a laser oscillator. Excimer lasers emit ultraviolet light which is highly absorbable into a silicon film, which is a typical semiconductor film, and are advantageous in terms of productivity because they are high-powered. However, they are very high-priced, their life is short, and their component parts need to be frequently changed. There is also a need to periodically change the gas necessary for oscillation. Maintenance of an excimer laser is time-consuming, so that the maintenance cost is considerably high. Therefore there is an urgent need for development of a laser annealing device to replace excimer lasers.
Various lasers were developed and improved in the latter half of 1990s. The demand for lasers has grown sharply. Among lasers newly developed. YAG lasers are regarded as suitable lasers for semiconductor film laser annealing. At the earlier stage of the process of putting semiconductor film laser annealing to practical use, there was a movement toward a use of YAG lasers for crystallization of semiconductor films. However, YAG lasers made room for excimer lasers because of their low output stability, their lower output level relative to that of excimer lasers, a need for conversion to a harmonic, etc.
In recent years, however, the output power of YAG lasers has been remarkably increased and the output stability has also been improved. Correspondingly, there has been a tendency to again make trials to apply YAG lasers to laser annealing. In the case of a use of a YAG laser for crystallization of a semiconductor film, there is a need for conversion to a harmonic according to the relationship with the absorption coefficient of the semiconductor film. However, a sufficiently high output level can be maintained even after conversion.
YAG lasers essentially have the advantages of maintainability, compactness and availability at a low price. Since YAG lasers are solid-state lasers and use no gas unlike excimer lasers, they are free from a need to change degraded excitation source components. It is said that the excitation sources (rods) of solid-state lasers has a life of 20 years or longer. Moreover, the number of component parts necessary for laser oscillation in YAG lasers is markedly smaller than that in excimer lasers.
YAG lasers also have a number of problems to be solved, although they have the above-described advantages. First, the oscillation frequency of YAG lasers is lower than that of excimer lasers, from which lower productivity results. This is because when the temperature of the rod of flashlamp-pumped YAG lasers is increased to an excessively high point, the thermal lensing effect becomes high to considerably worsen the laser beam shape and it is difficult to obtain a higher frequency. However, there is the prospect that this problem will be solved because laser diode-pumped YAG lasers capable of limiting the rise of the rod temperature have recently been developed.
Another problem of YAG lasers relates to the coherence of YAG lasers. Lasers in general have high coherence. Therefore, when a linear beam is obtained by the method of obtaining a beam having a uniform energy distribution, which is obtained by dividing a laser beam and by combining the divided beams, interference occurs in the linear beam to cause standing waves. Excimer lasers have a coherence length of several ten microns, which is extremely small in comparison with those of other lasers. In a linear beam obtained from an excimer laser, therefore, interference does not occur easily and standing waves thereby caused are not noticeable.
On the other hand, YAG lasers have a coherence length of about 1 cm, so that the standing waves caused as described above are considerably strong. FIG. 3 shows standing waves in a beam obtained by dividing a YAG laser beam into two beams and by combining the two beams. In FIG. 3 in which an energy distribution is imaged with a CCD camera, a pattern corresponding to a sine curve is clearly recognizable.
In view of the above-described circumstances, an object of the present invention is to uniformize a distribution of energy of a laser beam having high coherence. The present invention is particularly effective in uniformizing a distribution of energy of a laser beam with a comparatively long coherence length, e.g., a laser beam obtained by a YAG laser, a YVO4 laser or a YLF laser.
The present invention provides a method of manufacturing a semiconductor device by using a method for reducing interference fringes in a linear beam formed by a beam homogenizer.
A laser can produce a beam of linearly polarized light by making its polarization uniform. It is generally known that when laser beams having polarization directions perpendicular to each other are combined, no interference fringes occur. A beam of circularly polarized laser light may also be used. Beams of circularly polarized laser light do not interfere with each other if they differ from each other in the direction of rotation of circularly polarized light. Thus, beams of light having polarization directions independent of each other do not interfere with each other. The effect that the present invention is aimed to achieve can be obtained by utilizing this characteristic.
Therefore, if laser beams having polarization directions perpendicular to each other are combined to form a uniform beam, no interference occurs. A YAG laser or the like can emit linearly polarized laser light. If this layer beam is divided into two, and if a xcex/2 plate is inserted in the path for one of the divided two laser beams to rotate the polarization direction through 90xc2x0 while the other beam is made to travel directly, laser beams having polarization directions perpendicular to each other can be formed. This method enables only division of one beam into two and may be not sufficiently effective in making the resulting beam uniform. Therefore, this method is combined with some other method to increase the number of divisions to achieve sufficiently high uniformity.
To obtain a linear beam having a uniform energy distribution by using a laser having high coherence, it is desirable to obtain improved uniformity in each of the long-dimension direction and the short-dimension direction of the linear beam. Accordingly, it is preferable to combine beams divided into at least two in each direction, i.e., four divided laser beams, into one to form a uniform linear beam. Essentially, in forming a uniform linear beam, it is important to make the distribution of energy uniform in the long-dimension direction. This is because the uniformity in the long-dimension direction is directly reflected in the uniformity of laser annealing in the long-dimension direction. On the other hand, the uniformity in the short-dimension direction is not so important as that in the long-dimension direction. This is because the uniformity of laser annealing can be improved by finely overlapping, in the short-dimension direction, the areas irradiated with the linear beam. Consequently, uniformization in the short-dimension direction of the linear beam is effected by combining two laser beams having polarization directions perpendicular to each other, and uniformization in the long-dimension direction is effected by another method.
Even laser beams emitted from one light source can be combined without interference if they are combined by the optical path length equal to or larger than the coherent length. If this characteristic is utilized, three or more divided laser beams can be combined without interference to obtain a uniform beam. For example, optical path differences may be created by inserting in the optical path a block having high transmittance with respect the laser beam.
The optical path used in accordance with the present invention needs to have optical elements of extremely small aberrations, because high coherence of a laser beam results in a wavelike energy distribution under the influence of a spherical aberration, etc. FIGS. 6A to 6D show the distributions of energy of a laser beam from a YAG laser passed through various cylindrical lenses. xe2x80x9cFxe2x80x9d in FIGS. 6A to 6D denotes the ratio of the focal length of the lens to the diameter of the aperture of the lens. If F is smaller, the spherical aberration is larger.
FIG. 6A shows the distribution of energy of a laser beam of a YAG laser. This is a photograph of traces of direct irradiation of the laser beam on the amorphous silicon film. In the photograph shown in FIG. 6A, no noticeable energy nonuniformity is recognized. FIG. 6B shows a photograph of the energy distribution in similar irradiation of the amorphous silicon film with the laser beam of the YAG laser when the laser beam was passed through a cylindrical lens of F=7. A fringe pattern extending laterally is clearly recognizable. This is an energy distribution caused under the influence of a spherical aberration of the F=7 cylindrical lens. FIG. 6C shows the result when the beam was passed through a cylindrical lens of F=20, and FIG. 6D shows the result when the beam was passed through a cylindrical lens of F=100. The laser beam passed through the F=7 cylindrical lens was strongly influenced by the spherical aberration to cause a wavelike energy distribution. On the other hand, the laser beam passed through the F=20 cylindrical lens was not largely influenced by the spherical aberration and the resulting wavelike condition of the energy distribution was not noticeable. In the case of the laser beam passed through the F=100 cylindrical lens, no wavelike energy distribution was observed.
The F-number referred to in this specification is calculated by using as the lens aperture the area through which the laser beam passes actually. In a case where the size of the lens is larger than the size of the beam passing therethrough, the size of the beam is used as the aperture.
Structures in accordance with the present invention will be successively described below.
The structures of a beam processing method disclosed by the present invention relates to a method of processing a beam so that a distribution of energy of a laser beam having coherence is made uniform along or in the vicinity of an irradiation plane, the method comprising the steps of:
dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beams having polarization directions independent of each other;
combining the two laser beams into one on or in the vicinity of the irradiation plane;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
combining the plurality of laser beams into one on or in the vicinity of the irradiation plane.
In the above-described structure, the step of dividing the laser beam into two laser beams having polarization directions independent of each other may include using a xcex/2 plate.
Further, another structure of the present invention relates to a method of processing a beam so that a distribution of energy of a laser beam having coherence is made uniform along or in the vicinity of an irradiation plane, the method comprising the steps of:
dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beams having polarization directions perpendicular to each other;
combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
combining the plurality of laser beams into one on or in the vicinity of the irradiation plane.
Further, another structure of the present invention relates to a method of processing a linear beam so that a distribution of energy of a beam of linearly polarized laser light having coherence is made uniform along or in the vicinity of an irradiation plane, the method comprising the steps of:
dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beams having polarization directions perpendicular to each other;
combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a short-dimension direction;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
combining the plurality of laser beams into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a long-dimension direction.
In each of the above-described structures, the step of forming the laser beam having the polarization directions perpendicular to each other may include a use of a xcex/2 plate.
Further, another structure of the present invention relates to a method of processing a beam so that a distribution of energy of a beam of circularly polarized laser light having coherence is made uniform along or in the vicinity of an irradiation plane, the method comprising the steps of:
dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beam having circularly polarization directions independent of each other;
combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
combining the plurality of laser beams into one on or in the vicinity of the irradiation plane.
Further, another structure of the present invention relates to a method of processing a linear beam so that a distribution of energy of a beam of circularly polarized laser light having coherence is made uniform along or in the vicinity of an irradiation plane, the method comprising the steps of:
dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beam having circularly polarization directions independent of each other;
combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a short-dimension direction;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
combining the plurality of laser beams into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a long-dimension direction.
In each of the above-described structures, the step of forming the two laser beams having the circularly polarization directions independent of each other may include a use of a xcex/2 plate.
Further, in each of the above-described structures, the laser beam comprises one kind or a plurality of kinds of laser beams selected from laser beams emitted from a YAG laser, a YVO4 laser, and a YLF laser. If a plurality of kinds of laser beams are used, interference in the laser beam along or in the vicinity of the irradiation plane can be further reduced.
Further, in each of the above-described structures, the step of setting different optical path lengths in correspondence with the plurality of laser beams may include using a block having a high transmittance with respect to the laser beam.
Further, in each of the above-described structures, the step of dividing the laser beam into a plurality of laser beams includes a use of a cylindrical lens having an F-number of 20 or larger.
Further, in each of the above-described structures, the step of combining the plurality of laser beams includes a use of a cylindrical lens having an F-number of 20 or larger.
Further, a structure of a laser irradiation apparatus disclosed by the present invention relates to a laser irradiation apparatus for forming a laser beam having a uniform energy distribution along or in the vicinity of an irradiation plane, the apparatus comprising:
a laser oscillator for forming a laser beam having coherence;
means for dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the divided two laser beams having polarization directions independent of each other;
means for combining the two laser beams into one on or in the vicinity of the irradiation plane;
means for dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
means for combining the plurality of laser beams into one on or in the vicinity of the irradiation plane.
In the above-described structure, the means for dividing the laser beam into two laser beams having polarization directions independent of each other may include a xcex/2 plate.
Further, another structure of the present invention relates to a laser irradiation apparatus for forming a laser beam having a uniform energy distribution along or in the vicinity of an irradiation plane, the apparatus comprising:
a laser oscillator for forming a laser beam of linearly polarized laser light having coherence;
means for dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beams having polarization directions perpendicular to each other;
means for combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane;
means for dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
means for combining the plurality of laser beams into one on or in the vicinity of the irradiation plane.
Further, another structure of the present invention relates to a laser irradiation apparatus for forming a linear laser beam distributed along or in the vicinity of an irradiation plane, the apparatus comprising:
a laser oscillator for forming a laser beam of linearly polarized laser light having coherence;
means for dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beams having polarization directions perpendicular to each other;
means for combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a short-dimension direction;
means for dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
means for combining the plurality of laser beams into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a long-dimension direction.
In the each of the above-described structures, the means for forming the two laser beams having polarization directions independent of each other may include a xcex/2 plate.
Further, another structure of the present invention relates to a laser irradiation apparatus for forming a laser beam having a uniform energy distribution along or in the vicinity of an irradiation plane, said apparatus comprising:
a laser oscillator for forming a laser beam of circularly polarized laser light having coherence;
means for dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beam having circularly polarization directions independent of each other;
means for combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane;
means for dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
means for combining the plurality of laser beams into one on or in the vicinity of the irradiation plane.
Further, another structure of the present invention relates to a laser irradiation apparatus for forming a linear laser beam distributed along or in the vicinity of an irradiation plane, the apparatus comprising:
a laser oscillator for forming a laser beam of circularly polarized laser light having coherence;
means for dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the two laser beam having circularly polarization directions independent of each other;
means for combining the two laser beams divided in the first direction into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a short-dimension direction;
means for dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction, the plurality of laser beams having optical path lengths different from each other; and
means for combining the plurality of laser beams into one on or in the vicinity of the irradiation plane to make the distribution of energy of the linear laser beam uniform in a long-dimension direction.
In each of the above-described structures, the laser oscillator is one kind or a plurality of kinds selected from the group consisting of a YAG laser, a YVO4 laser, and a YLF laser.
Further, in each of the above-described structures, the means for dividing the laser beam into a plurality of laser beams comprises a cylindrical lens having an F-number of 20 or larger.
Further, in each of the above-described structures, the means for combining the plurality of laser beams includes a use of a cylindrical lens having an F-number of 20 or larger.
Further, a structure of a manufacturing method of a semiconductor device disclosed by the present invention relates to a method of manufacturing a semiconductor device having a TFT formed on a substrate, the method comprising the steps of:
forming a non-single crystalline semiconductor film over the substrate;
irradiating a non-single crystalline silicon film with a linear beam while moving the beam relative to the semiconductor film; and
forming the linear beam, the step of forming the linear beam including;
oscillating a laser beam having coherence;
dividing the laser beam into two laser beams in a first direction perpendicular to the direction of traveling of the laser beam, the divided two laser beams having polarization directions independent of each other;
combining the two laser beams into one on or in the vicinity of the irradiation plane;
dividing the laser beam into a plurality of laser beams in a second direction perpendicular to the direction of traveling of the laser beam and also perpendicular to the first direction; and
combining the plurality of laser beams into one on or in the vicinity of the irradiation plane to form the linear laser beam having a long-dimension direction parallel to the second direction.
In each of the above-described structures, the laser oscillator is one kind or a plurality of kinds selected from the group consisting of a YAG laser, a YVO4 laser, and a YLF laser.
Further, in the above-described structure, a method of forming the linear laser beam comprises a cylindrical lens having an F-number of 20 or larger.
As described above, the present invention makes it possible to effectively improve the uniformity of the distribution of energy of a laser beam having coherence by reducing the coherence of the laser beam. If a combination of the present invention and a solid-state laser is used in the process of crystallizing a semiconductor film, a remarkable reduction in manufacturing cost can be expected. Also, suitable operating characteristics and sufficiently high reliability can be achieved in electro-optic devices and semiconductor devices fabricated by making TFTs on the thus-obtained semiconductor film and by using the TFTs, which devices are typified by an active-matrix liquid crystal display device.